I’m starting with a regular &mut ThreadRng in all my functions, but it limits dependency injection capacities. There are SeedableRng ofc, but how to use it in places where initially it was a ThreadRng ?

Coming from other languages I’d like to specify some interface in the fn declaration, but &mut dyn Rng doesn’t work because size is not defined at compile-time.

What I ended up with is passing &mut StdRng everywhere and

let mut rng = match config.seed {
  Some(seed) => StdRng::seed_from_u64(seed),
  _ => StdRng::from_rng(rand::thread_rng()).unwrap_or(StdRng::from_entropy())
};

But it’s awkward to init 1 Rng only to setup another Rng

I also searched the pattern StdRng::from_rng(rand::thread_rng()) and it’s just 56 samples in github, from 141k of using rand::thread_rng()

So what’s the best signature for a fn accepting Rng and the best way to init with an optional seed?

p.s. I don’t care about wasm/etc runtimes, it’s just a regular server-side app